1. Field of the Invention
The present invention relates to a display control apparatus and method for a flat panel display having lines of pixels arrayed in a horizontal scanning direction and, more particularly, to a display control apparatus and method used for driving the display by means of a video signal of a scheme which has a larger number of horizontal picture signals per frame than the number of the horizontal pixel lines.
2. Description of the Related Art
The NTSC (National Television System Committee) scheme and the PAL (Phase Alternation by Line) scheme are known as video signal standards used in color television sets using CRT displays in which interlace scanning is performed. The NTSC scheme has a one-frame image display period of 525H (H: horizontal scanning period) equal to 1/30 sec. The PAL scheme has a one-frame image display period of 625H (H: horizontal scanning period) equal to 1/25 sec.
FIG. 1 shows an NTSC video signal for a one-frame image, and FIG. 2 shows a
video signal for a one-frame image. In interlace scanning, horizontal scanning lines are sequentially drawn to scan an odd field on the CRT screen, and then sequentially drawn to scan an even field on the CRT screen. After scanning each field, the current scanning position is returned to a scanning start position for the next field within a vertical blanking period TG shown in FIGS. 1 and 2. Since the vertical blanking period TG and other sync. control periods must be included in the one-frame image display period, this restricts the number of horizontal scanning lines which can be actually drawn on the screen in accordance with each video signal VS. Therefore, the number of effective horizontal picture signals Sm is reduced to about 470 in the NTSC video signal, and to about 570 in the PAL video signal.
A flat panel display such as a liquid crystal display has a plurality of horizontal pixel lines, which are equivalent to the horizontal scanning lines on the CRT screen. In display control of the liquid crystal display, all the horizontal pixel lines are driven within a field scanning period by sequentially selecting pairs of adjacent horizontal pixel lines and supplying a horizontal picture signal to a selected pair of horizontal pixel lines. This control is performed to remedy a problem that the image quality is lowered as compared with a CRT display when half of the horizontal pixel lines are driven within the field scanning period.
Liquid crystal displays have a greater advantage in portability than CRT displays. However, when a liquid crystal display is brought to a country where the PAL scheme is determined as the domestic video signal standard, the display would not operate normally. A PAL video signal has about 570 horizontal picture signals, which is remarkably larger in number than the 470 horizontal pixel lines provided in a typical NTSC liquid crystal display. Since shortage of horizontal pixel lines arises, it is difficult to display images without deleting an upper or bottom portion thereof.
Conventionally, the following display control apparatuses have been developed to compensate for a difference between the number of horizontal picture signals in a PAL video signal and the number of horizontal pixel lines in a liquid crystal display.
The first display control apparatus stores a PAL video signal for one frame in a frame memory (or line memories), and performs interpolation and decimation on the horizontal picture signals contained in the stored PAL video signal by using a digital processor in order to reduce the number thereof to about 235 per field. In a field scanning period, each of the remaining horizontal picture signals is supplied to a corresponding pair of horizontal pixel lines. However, the manufacturing cost of the apparatus is increased due to the use of the frame memory and the digital processor.
The second display control apparatus uses all the horizontal picture signals contained in a PAL video signal by assigning five horizontal pixel lines in the liquid crystal display to three horizontal picture signals. Therefore, while all the horizontal pixel lines are sequentially selected, the first horizontal signal is supplied to the first and second pixel lines, the second horizontal picture signal is supplied to the third and fourth pixel lines, and third horizontal picture signal is supplied to the fifth pixel line, for example. Since no horizontal picture signal is deleted in the second display control apparatus, this apparatus can display more natural images as compared with the first display control apparatus. However, the second display control apparatus has a configuration in which an extra line memory or the like must be provided. This increases the manufacturing cost of the apparatus.